I like Camelbak the best. The packs themselves are virtually indestructible, and the bladder and bite valves work well.
I also have an Osprey hydration system. I like the way it fits better than the Camelbak does, and it has some cool features, but there are some things about it that aren't very good. There is a lifetime warranty on the pack itself, and they'll repair it with no questions asked, but you have to send it in and be without out it for a few weeks.
My Osprey pack has two elastic loops on the shoulder strap that the hydration line is routed through. After only one week of using the pack, one of the loops tore out of the shoulder strap. Osprey fixed it, and a week later it tore out again.
Another issue I have with this pack is with the bladder. It has a ridged support on the outside, which makes sliding it into the pack easy, but at the same time it makes getting the air out of the bladder hard. The bite valve also has an issue. It leaks. It constantly drips water onto my leg while I'm riding. I either have to turn the valve off, or blow back into the line to push the water back into the bladder.